I would agree if this were a deep sand bed, but with a shallow bed, where
will
the detritus go?

to the exact same place it would go in a DSB, the processing of detritus only
requires
critters and those live in the top 1" or less of a DSB, Nitrate removal is a
different
story, but its not in the question

IME this is not true.. there have been many occasions where I have had multiple
tanks chained together on the same systems.. and the only place detritus wuold
build is in the tanks that had shallow sand beds. Probably because not as many
critters/worms inhabit shallow beds as much as they do when the sand bed is
deeper. It may also be due to the natural carbon cycle and small pockets of
sulfide. .. I dunno, just my experience..
Rod Buehler
www.asplashoflife.com